Was installing xfce from ports, and when compiling thunar as prerequisite, got this warning:

... [snip] ...
checking sys/mman.h presence... yes
checking for sys/mman.h... yes
checking sys/mount.h usability... no
checking sys/mount.h presence... yes
configure: WARNING: sys/mount.h: present but cannot be compiled
configure: WARNING: sys/mount.h:     check for missing prerequisite headers?
configure: WARNING: sys/mount.h: see the Autoconf documentation
configure: WARNING: sys/mount.h:     section "Present But Cannot Be Compiled"
configure: WARNING: sys/mount.h: proceeding with the preprocessor's result
configure: WARNING: sys/mount.h: in the future, the compiler will take precedence
configure: WARNING:     ## ---------------------------------------- ##
configure: WARNING:     ## Report this to http://bugzilla.xfce.org/ ##
configure: WARNING:     ## ---------------------------------------- ##
checking for sys/mount.h... yes
checking sys/param.h usability... yes
checking sys/param.h presence... yes
checking for sys/param.h... yes
checking sys/resource.h usability... yes
checking sys/resource.h presence... yes
checking for sys/resource.h... yes

... [/snip] ...